Clock synchronisation between devices using message timestamps

Abstract

A method at a first device for synchronising a first clock of the first device to a second clock of a second device, includes receiving a first message comprising an identifier from a third device; generating a first timestamp in dependence on the time at which the first message is received at the first device according to the first clock; receiving a second message from the second device comprising the identifier and a second timestamp, the second timestamp having been generated in dependence on the time at which the second device received the first message from the third device according to the second clock; and adjusting the first clock in dependence on a time difference between a time indicated by the first timestamp and a time indicated by the second timestamp.

What is claimed is: 
     
       1. A first device comprising a first clock, the first device for enabling synchronisation of a second clock at a second device with the first clock, the first device being configured to:
 receive a first message comprising an identifier from the second device; 
 generate a first timestamp in dependence on the time at which the first message is received at the first device according to the first clock; 
 store in memory the identifier and the first timestamp; 
 receive a second message from the second device comprising the identifier and a second timestamp, the second timestamp having been generated in dependence on the time at which the second device transmitted the first message to the first device; and 
 in response to receiving the second message, retrieve the first timestamp stored in association with the identifier, 
 adjust the first clock in dependence on a time difference between a time indicated by the first timestamp and a time indicated by the second timestamp. 
 
     
     
       2. The first device as claimed in  claim 1 , the first device being further configured to:
 receive a plurality of identifiers and store each of the identifiers in memory; 
 for each of the plurality of identifiers, store in memory an associated timestamp indicating a time at which that identifier was received at the first device according to the first clock. 
 
     
     
       3. The first device as claimed in  claim 1 , wherein the first message is broadcasted by the second device. 
     
     
       4. The first device as claimed in  claim 1 , the first device being further configured to:
 receive media and a media playback time from the second device; and 
 play out the media in dependence on the media playback time according to the adjusted first clock. 
 
     
     
       5. A first device comprising a first clock, the first device for enabling synchronisation of a second clock at a second device with the first clock, the first device being configured to:
 transmit a first message comprising an identifier to the second device; 
 generate a timestamp in dependence on the time at which the first message is transmitted from the first device according to the first clock; 
 generate a second message comprising the identifier and the generated timestamp; and 
 transmit the second message to the second device; 
 determine an event time in dependence on a time according to the first clock and an offset period of time; and, 
 transmit the determined event time to the second device. 
 
     
     
       6. The first device as claimed in  claim 5 , wherein the event time comprises a playback time for playing media. 
     
     
       7. The first device as claimed in  claim 5 , further comprising a third clock, wherein the identifier is generated in dependence on the third clock. 
     
     
       8. The first device as claimed in  claim 7 , wherein the first clock is configured to operate to a first clock resolution and the third clock is configured to operate to a second resolution lower than the first resolution. 
     
     
       9. The first device as claimed in  claim 7 , further comprising a communications interface comprising the third clock, the third clock being a physical layer clock. 
     
     
       10. The first device as claimed in  claim 5 , wherein the first message is transmitted directly to the second device. 
     
     
       11. The first device as claimed in  claim 6 , the first device configured to transmit the media to the second device. 
     
     
       12. The first device as claimed in  claim 5 , the first device being configured to:
 transmit the first message to a third device; 
 determine a further event time in dependence on a time according to the first clock and an offset period of time; and, 
 transmit the determined further event time to the third device. 
 
     
     
       13. The first device as claimed in  claim 12 , wherein the further event time comprises a further playback time for playing media. 
     
     
       14. The first device as claimed in  claim 5 , the first device configured to receive an acknowledgement message from the second device, the acknowledgement message comprising an indication that the first message was received by the second device. 
     
     
       15. The first device as claimed in  claim 5 , the first device configured to:
 receive a third message comprising a second identifier from a fourth device; 
 generate a second timestamp in dependence on the time at which the third message is received at the first device according to the first clock; 
 generate a fourth message comprising the identifier and the generated second timestamp; and, 
 transmit the fourth message to a fifth device. 
 
     
     
       16. The first device as claimed in  claim 15 , wherein the first and second messages are transmitted in a first communications mode and the third and fourth messages are received and transmitted in a second communications mode. 
     
     
       17. The first device as claimed in  claim 5 , the first device configured to:
 receive a third message comprising a second identifier from a fourth device; 
 generate a second timestamp in dependence on the time at which the third message is received at the first device according to the first clock; 
 receive a fourth message from a fifth device comprising the second identifier and a third timestamp, the third timestamp having been generated in dependence on the time at which the fifth device received the third message from the fourth device; and, 
 adjust the first clock in dependence on a difference between a time indicated by the second timestamp and a time indicated by the third timestamp. 
 
     
     
       18. The first device as claimed in  claim 17 , wherein the first and second messages are transmitted in a first communications mode and the third and fourth messages are received in a second communications mode. 
     
     
       19. The first device as claimed  claim 5 , wherein the first device is configured to operate according to a WLAN standard and the identifier is a timing synchronisation function. 
     
     
       20. A non-transitory computer readable storage medium having stored thereon computer readable instructions that, when executed at a computer system, cause the computer system to perform a method for synchronising a first clock of a first device to a second clock of a second device, the method comprising:
 receiving at the first device a first message from the second device; 
 generating a first timestamp in dependence on the time at which the first message is received at the first device according to the first clock; 
 storing in memory the identifier and the first timestamp; 
 receiving at the first device a second message from the second device comprising the identifier and a second timestamp, the second timestamp having been generated in dependence on the time at which the second device transmitted the first message to the first device; and 
 in response to receiving the second message, retrieving the first timestamp stored in association with the identifier, 
 adjusting the first clock in dependence on a time difference between a time indicated by the first timestamp and a time indicated by the second timestamp.